Maybe I should have posted this in motorsport section but it's brake related.

I'm having a hard brake pedal issue at autocross after a couple of hard braking. The foot pressure required to stop the car increases dramatically but the stoping power remains the same, it's hard to predict in a racing situation without loosing some time.

This week end at Fingerlakes pro solo I confirmed that it was not pad, lines, fluid, or rotor related as everything was fresh. I'm thinking it's vaccum or booster related, maybe I'm needing more vaccum than the system is creating or have a weak booster.

Do any of you guys had this problem or could provide inputs?
Remember it only happen in sustained hard (left foot braking) situations so keep regular driving stories away. Thanks

 

Perhaps there is a slight leak in the brake booster or the check valve is leaking very slowly? Both slight leaks wouldn't necessarily show up with daily driving but extended left foot braking on the track would see boost on one side the check valve and vacuum on the other. Of course doing left foot braking would most likely have the engine producing some boost (or at least no vacuum) so your braking power wouldn't be replenished.

 

Thanks, I'm going to go through the system and make sure it's not leaking on me. However, you raise a good point that because of the left foot braking done while still on boost, there might not be enough vaccum available to fully assist the booster. I will have to ask somebody who co-drove my car if they had similar issues since they all right foot brake. Now the question is if any other left foot brakers here have had similar experiences, I know there must be some L.F.B.ers on the forum.

 

I've never LFB in an EVO (yet), but my DSM has no such issue even with long LFB sessions. I really would expect to find some small leak in the system.

 

Lfb Ftw!!!

 

Find a place to put one of these or stop the LFB.

http://www.dougherbert.com/camsvalve...-c-83_348.html

 

I think the vaccum reserve can is a better proposition as I need the LFB to free up the right foot so I can avoid falling off boost on the slow stuff. Thanks for the link, and would you happen to know how big this thing is?

 

The one from Crane is
Diameter (in): 5.000 in.
Length (in): 7.000 in.

 

cfdfireman1, you are the man .

 

How do I go about checking the booster and check valve? I get long pedal travel (have to push pretty far) in order for the brakes to lock up. Also, there is no brake feel - it all feels the same pretty much before and during lock-up (abs). My pedal is just soft and mushy, even with Motul.

 

stockEvoIX

Sounds like you have air in the system bleed them again. r/r l/f l/r r/f then repeat. The type of pad you're using will affect pedal feel also but it should never feel mushy.

 

Thanks cfdfireman! Yeah, it has good feel after a few pumps so hopefully a bleed will make it good. I'm putting in Axxis Ultimates in the front so we'll see how it goes.

 

..do a full flush of the system if the fluid is more than a year old and depending on how hot you got the brakes like hard driving or a track day.

 

what's the difference between a bleed and a flush? thanks

 

Bleed is very limited and just makes sure there is no air in the system a flush is like an oil change and replaces all the brake fluid with new. A full flush will take take like 2 pints at least to do.